Numerous commercially valuable plants, including common agricultural crops, are susceptible to attack by insect and nematode pests. These pests can cause substantial reductions in crop yield and quality. Traditionally, farmers have relied heavily on chemical pesticides to combat pest damage. However, the use of chemical pesticides raises its own set of problems, including the cost and inconvenience of applying the pesticides. Furthermore, chemical residues raise environmental and health concerns. For these and other reasons there is a demand for alternative insecticidal agents.
An environmentally friendly approach to controlling pests is the use of pesticidal crystal proteins derived from the soil bacterium Bacillus thuringiensis (“Bt”), commonly referred to as “Cry proteins.” Many of these proteins are quite toxic to specific target insects, but harmless to plants and other non-targeted organisms. Some Cry proteins have been recombinantly expressed in crop plants to provide pest-resistant transgenic plants. Among those, Bt-transgenic cotton and corn have been widely cultivated.
A large number of Cry proteins have been isolated, characterized and classified based on amino acid sequence homology (Crickmore et al., 1998, Microbiol. Mol. Biol. Rev., 62: 807–813). This classification scheme provides a systematic mechanism for naming and categorizing newly discovered Cry proteins.
It has generally been found that individual Cry proteins possess relatively narrow activity spectra with the exception of Cry2A. Cry2A is unusual in that this subset of Cry proteins possesses a broader effective range that includes toxicity to both the Lepidoptera and Diptera orders of insects. The Cry2A protein was discovered to be a toxin showing a dual activity against Trichoplusia ni (cabbage looper) and Aedes taeniorhynchus (mosquito) (Yamamoto and McLaughlin, 1982, Biochem. Biophys. Res. Comm. 130: 414–421). The nucleic acid molecule encoding the Cry2A protein (termed Cry2Aa) was cloned and expressed in B. megaterium and found to be active against both Lepidoptera and Diptera insects (Donovan et al. 1988, J. Bacteriol. 170: 4732–4738). An additional coding sequence homologous to Cry2Aa was cloned (termed Cry2Ab) and was found to be active only against Lepidoptera larvae (Widner and Whiteley, 1989, J Bacteriol 171:2).
Second generation transgenic crops could be more resistant to insects if they are able to express multiple and/or novel Bt genes. Accordingly, new insecticidal proteins having broad activity spectra would be highly desirable.